A. Field of the invention
A frequency counter, being a digital instrument, is limited in its range by the speed of its logic circuitry. Today the state of the art at high-speed logic allows the construction of direct (without any pre-scaling) digital frequency counters in IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT--December 1977--Volume I.M.26--Number 4--p.p. 396-402: DEVELOPING PROGRAMMABLE BROAD-BAND COUNTING CIRCUITS FOR HIGH-SPEED DATA TRANSMISSION AND FREQUENCY DIVISION by A. PAPHITIS.
Using down conversion techniques, frequency measurement extension up to 40 GHz is made possible.
In conventional counters, however, accuracy drops with input frequency if the gate period, during which the measurement occurs, is held constant. With low frequencies absurdly long measurement times would be needed to keep resolution.
To measure a frequency of IHz with 1% precision, for instance, the gate period should be larger than 100 seconds.
The method used to shift a given frequency of unknown value to, in the case of low frequency counters, higher frequency values that can be measured, fast and precisely, can be used in other fields too. A typical application would be a digital mixer, or modulator and demodulator, system as the product Nf.sub.1 can be interpreted as the product of two frequencies f.sub.1 and f.sub.2 where N=f.sub.2.
B. Description of the Prior Art
Of the various methods to get around the problem of low frequency counters, the most notable are reciprocal counters (R.C.'s) and counters using PHASE-LOCKED LOOPS (PLL's). R.C.'s measure the period and then arithmetically convert to get the frequency. R.C.'s presently work to 500 MHz, but the advantage of constant resolution disappears when the input frequency exceeds the reciprocal unit clock frequency, usually 10 MHz, and a second counter or a higher unit clock frequency is needed to measure such frequencies accurately, both being extremely uneconomical. R.C.'s are very complex circuits. Several counters of various stages, depending on frequency range and accuracy requirements to be covered, are needed. As described for instance in ELECTRONICS, Mar. 4, 1976, p.p. 100-101: COUNTER INVERTS PERIOD TO MEASURE LOW FREQUENCY by Matthew L. Fichtenbaum, several counter stages are needed to measure the period of the input signal by counting the number of unit clock frequency pulses, N, in one period and inverting the result. While with a conventional counter a 1% accuracy is obtained without need of changing the frequency range, for frequencies above 100 Hz and a gate time of 1 second, this is not the case in R.C.'s. To measure, for instance 100 KHz with the above mentioned unit clock frequency of 10 MHz, 100 pulses of the clock frequency are registered. To measure 100 Hz with the same frequency would require a five stage decimal counter merely to count the period duration. Inversion is now performed by dividing a number K (which is much larger than N if the lowest frequency to be counted is for instance 0.01 Hz, then N=10.sup.9) by the count of N where a bit comparator of several stages (if N=10.sup.9, nine stages) to compare the period count with the count of another auxiliary counter of equal length (in the example, nine stages) and a third counter (of more than nine stages) to count K are necessary and lastly, is necessary too, the readout display counter. Several techniques are known to reduce this effect but the main problem remains the same, which ends always in a big implementational effort. Further another serious disadvantage is the loss of information since, during inversion, the counter input is disabled. As we shall see later it is important to note that this is the case even if the input frequency did not change or presents slight change.
Consequently a frequency input signal may not be continuously monitored which is important for instance in ratemeters (heart rate monitoring, motor regulation applications, car velocity and acceleration control, etc.)
Of the many known low frequency measurement methods, the R.C.'s have been mentioned because of their popularity among instrument manufacturers. It should be mentioned, however, that the PLL approach, although versatile, has its share of problems. The frequency limit here is 100 MHz and it is obvious that it is easier to build a 100 Mhz pulse counter than to implement a 100 MHz PLL. This, however, is not the main difficulty. PLL's tend to lock on subharmonics and behave strangely with narrow pulses and modulated FM and AM signals. Because of high frequency limitations of LSI, PLL's will probably never reach frequencies much higher than 100 MHz. Last but not least, PLL prices are still high.
When measuring events between two time marks (or time intervals between two events) several error terms add together, for example, time base error, trigger error, etc. as described in ELECTRONIC DESIGN 24, Nov. 22, 1974, p.p. 162-167: MEASURE TIME INTERVAL PRECISELY by David Martin. One of the error sources described in the above mentioned work is the so-called .+-.1 count. This refers in the case of frequency (and not period) measurement to the fact that pulse counting always provides this inherent uncertainty, since there can usually be no guarantee of coherence between the input frequency and the gate time (in measuring periods, there is no coherence between input frequency and counted clock).
One of the consequences of the .+-.n count ambiguity (which can be greater than .+-.1) is that frequency counters are unable to register fractions of a pulse. Precision drops drastically with input frequency. A high frequency counter of a 1 Hz resolution at 100 MHz means a .DELTA.f/f=1 Hz/10.sup.8 Hz=10.sup.-8. For the same counter at 100 Hz we have:
ti .DELTA.f/f=1/100=10.sup.-2.
Another important factor in counting pulses is the short term stability of the time base oscillator, related with fast phase fluctuations and random frequencies generated along with the desired frequency. The less time the measurement takes, the greater the uncertainty (for times of one second and less). Measuring very short periods, there is this additional limitation, forcing averaging, by measuring more than one event and dividing by the number of measurements to obtain the result, such method meaning loss of resolution.